Power plant installations, in which large-volume steam turbines are used, are used inter alia for the local supply of power. The steam turbines used in such power plants have relatively high masses and are generally configured for a predefined rated power. These power plants, which may also be termed conventional power plants, may in a first approximation be split into pure steam power plants on one hand and gas and steam power plants on the other. Both share the fact that fossil fuels are required in order to generate electrical energy. Such power plants were hitherto conceived and configured for a base load. As a consequence of the increasing proportion of renewable energy sources—such as wind energy—which are largely impossible to control, the abovementioned conventional power plants must ever more frequently be operated at partial load. This means that the power plants do not supply the rated power for long periods, but rather supply a percentage of the rated power as partial load. The partial loads may, in some cases, be for example 25% of the full load.
This means that these power plants must be operated flexibly, wherein the change from comparatively low partial load to full load should occur as quickly as possible and without there being a limit on the number of load changes. The problem with that is that the temperature of the steam leaving the reheater unit drops markedly under extreme partial load, such as 25%, due to the lower availability of heat from the cooler flue gas. This temperature drop can be up to 60° Kelvin. However, these temperature variations are also transmitted to the components. This means that, in less-than-ideal cases, the voluminous and massive components have to be constantly heated and cooled. Thick-walled components in particular, such as an intermediate-pressure turbine section shaft, may be heated only comparatively slowly while observing desired changes in load. However, this runs counter to the requirement of switching the power plant from extreme partial load to full load in the shortest possible time.
For this reason, the reheater heating surfaces have hitherto been oversized and the hot reheater temperature in the upper load region, for example between 70% and 100%, has been controlled taking into account the thermodynamic efficiency losses resulting therefrom. The hot reheater temperature, which prevails downstream of the reheater unit, is referred to as “hRH”. A further approach consists in imposing appropriate limits on the load gradients in the lower load region, or in reducing the permissible load changes, wherein increased wear is also taken into account, such that the thick-walled components have to be exchanged early.